Our overall objective is to study the genetic, molecular and cellular basis for control of cell proliferation in human cells. Based on our previous studies of the regulation of DNA synthesis in heterokaryons formed between replicative cells and non-replicative cells, we have formulated a hypothesis that cessation of proliferation in normal human cells is governed by an inhibitor of entry into S phase. Cells transformed by DNA tumor viruses appear to be able to override this inhibitor, whereas carcinogen-transformed cells and most human tumor cells are still sensitive to the inhibitor. In this proposal we will test several of the tenets and predictions of this hypothesis, e.g., whether normal cells produce the inhibitor whenever they experience culture conditions that are inadequate for proliferation, whether carcinogen-transformed cells and human tumor cells retain the ability to make the inhibitor, whether SV40 large T antigen and/or small t antigen are responsible for the ability of SV40-transformed cells to override the inhibitor, etc. We will also look for the molecular entity that corresponds to the putative inhibitor. In the second part of this project, we will study the complexity of the genetic basis for control of proliferation in normal and transformed human cells. We will study the following normal phenotypes: finite proliferative lifespan (FPL+), senescence (S+), quiescence (Q+) and anchorage dependence (AD+), and their transformed counterparts FPL-, S-, Q- and AD-. We will carry out dominance tests, complementation tests and segregation analyses using normal human fibroblasts, carcinogen-transformed human fibroblasts and selected human tumor cells lines. These three types of genetic analysis will help us to understand how many genes are involved in expression of the FPL+, S+, Q+ and AD+ phenotypes, which such genes are dominant, which transformed cells share an alteration in the same gene and whether there may be a common gene involved in the expression of more than one of these phenotypes. These studies arte directed toward the ultimate goal of understanding the molecular basis for human cancer and aging in the hope that this will lead to better ways to prevent or cure cancer and age-related disabilities.